The invention relates to a generator/motor system, in particular for application in mobile units, motor vehicles, ships and the like as an on-board power system generator and starter, of the type having a rotational field machine with three generator phase windings and a pulse-controlled inverter that has a predetermined maximum power and is connected to the three generator phase windings of the rotational field machine. The present invention also relates to a method for operating this generator/motor.
At present, efforts are being made in motor vehicles with an internal combustion engine to combine the starter and generator to form a single electric machine.
However, during these efforts there is a problem, even at the general design stage, that two completely contradictory requirements have to be met.
On the one hand, in order to start and speed up an internal combustion engine it is necessary to apply an extremely high turning torque. This torque may be, depending on the engine capacity or cylinder number of the internal combustion engine, greater than 240 Nm. Furthermore, the electric machine must also be able to provide torque reserves for speeding up the internal combustion engine to the starting speed.
On the other hand, after the internal combustion engine has been successfully started, the electric machine which is designed as a starter/generator should operate predominantly as a generator in order to feed into the on-board power system of the motor vehicle. In this context, there is a need for a constant output of power over the extremely spread out rotational speed range, predefined by the internal combustion engine, from 600 to 6000 1/min (motor) with the highest possible efficiency.
It is virtually impossible to meet both requirements economically with a standard drive composed of a three-phase rotational field machine 30 and voltage-impressing pulse-controlled inverter (PRW) 31 in a rotational field bridge circuit with filter capacitor C, as shown in FIG. 3A.
A problem which it is necessary to overcome in this context is the necessary miniaturization and complete integration of the power electronics. The necessary filter capacitors are an impediment to integration. In particular, given the relatively low on-board power system voltage of 42 V, phase currents of approximately 1200 A in asynchronous machines are currently under discussion in order to generate the starting torques which are required. The intermediate circuit capacitors C, such as are shown, for example, in FIG. 3A which shows the design of a conventional drive system with rotational field machine 30, pulse-controlled inverter 31 and intermediate circuit capacitor C, assume considerable dimensions in this context, and these dimensions are an impediment to integration.
Furthermore, first measurements in the absorber space have shown that it is not possible to make any compromises here. The filtering is necessary in order to fulfill the stringent EMC requirements in motor vehicles. It is imperative to reduce the currents of the machine and thus the filter currents while keeping the other properties of the drive the same.
The configuration of the drive system with a rotational field machine and pulse-controlled inverter is conventionally as follows, described with reference to FIG. 3B.
FIG. 3B shows a conventional rotational speed/torque characteristic. The continuous line in FIG. 3B shows what can be achieved with a specific configuration of the rotational field machine and an associated pulse-controlled inverter power.
If, for example, the starting torque is to be increased while retaining the standard pulse-controlled inverter topology, i.e. one pulse-controlled inverter in a six pulse bridge circuit, and retaining the pulse-controlled inverter (apparent) power, the winding of the rotational field machine must be correspondingly changed. In the simplest case, more turns with thinner wires are formed. This leads to the characteristic curve shown by dashed lines in FIG. 3B. It is apparent that although this measure can increase the starting torque with an unchanged pulse-controlled inverter power, this can only be achieved at the cost of the generator power at relatively high rotational speeds. The configuration point drops correspondingly. Owing to the relatively high number of turns, the rotational field machine reaches its field weakening mode, i.e. the modulation limit of the pulse-controlled inverter, earlier and is able to output less power later during the generator mode.
In particular in motor vehicle applications, and specifically starter/generator arrangements, the costs for the pulse-controlled inverter also play a decisive role. The costs of a pulse-controlled inverter are nowadays no longer assessed very much according to the current strength which the pulse-controlled inverter has to bear but rather according to the current strength which has to be commutated in the topology. This characteristic variable determines the filter expenditure which has to be made particularly in the especially EMC-sensitive field of the car industry. In addition, the filters are an impediment to miniaturization, as are in particular also the reliability problems at high temperatures. For this reason it is necessary to attempt to configure the power electronics in the drive circuit as efficiently as possible, in particular to reduce the currents to be commutated.
M. Osama, T. A. Lipo “Modeling and analysis of a wide-speed-range induction motor drive based on electronic pole changing”, IEEE Transactions on Industry Application, Vol. 33, No. 5, September/October 1997 describes a rotational field machine with poles which can be switched over, two winding systems and two separate pulse-controlled inverters. However, less than optimum winding factors are obtained with the specific combination of the winding systems so that the rotational field machine cannot convert the maximum possible pulse-controlled inverter current for a given overall size of the pulse-controlled inverter into the torque in an optimum way. The dynamic behavior when the rotational field machine is switched over is not possible without corresponding torque transient effects, which can throw up particular problems in the drive phase, which can adversely affect the user's comfort. The Dahlander circuit which has also been known for a long time has a similar problem.
In DE 199 31 010 A1, a so-called diode-clamp double-three-level converter, which is known per se, is actuated by a novel pulse method in such a way that a parallel/serial switchover of the two winding systems can be brought about. At the same time, the number of poles of the rotational field machine can be retained during switching over. Since the switching over is brought about by a different predefinition of the voltage vectors, the switching over also takes place with little noise and without torque transient effects. In addition, the winding systems can also be “pivoted” in their phase so that a further significant reduction in the intermediate circuit current to be filtered can be brought about. Although this system is most developed technically, it is very complicated and costly.
For this reason, a system with a feed power converter and a machine power converter, that is to say a genuine converter, would be more suitable since then a higher degree of flexibility can be achieved. Such a genuine converter is described, for example, in L. Sack, “Reduction of losses in the DC-link capacitor of two-stage self-commutated converters”, Proceedings of the EPE '99, Lausanne, Switzerland. In said document it is also possible to achieve a significant reduction in the ripple current to be filtered by synchronizing the pulse patterns. If it is possible to reduce the ripple currents, the efficiency of the system can also be increased simultaneously since a relatively large amount of energy at the feed power inverter of the capacitor is conventionally also converted into dissipated energy.
For this reason, the object of the present invention is to construct a generator/motor system and a method for operating this motor/generator system in which and with which the currents which are to be commutated in the pulse-controlled inverter can be significantly reduced in a simple and cost-effective way.